OK, lead and lead change both terms used for cantering/loping, depending on which saddle you are using, for our purposes both are the same.
Definition of canter, stolen from an online dictionary 1.
A smooth three-beat gait of a horse that is slower than a gallop but faster than a trot, in which the feet touch the ground in the three-beat sequence of near hind foot, off hind foot and near front foot, off front foot.
Actually that is confusing, got to love the internet...read this http://www.equusite.com/articles/bas...icsGaits.shtml
it shows you the pattern and footfalls for the canter.
Now when we talk about lead, most people are talking about the inside front leg, now while this is not technically correct, as the lead starts from the back, it is so much easier to see the front leg...so I am looking at which front leg reaches forward, that is the "leading leg'
Now if you are riding in an arena this is important, because you want the inside leg to be the leading leg, it keeps your horse balanced. Now to get that you have to remember that is is actually the outside hind that takes the first canter step, so you slide your outside leg a little further back, and squeeze, at the same time raising for inside hand a little to support that shoulder. Now most well trained and generous horses will automatically give you the correct lead, but some struggle on one side or the other, Gibbs could not pick up a right lead first time, so I would hear "WRONG LEAD" usually before I picked it up, so I would have to go back to the trot and try again, or change the lead..
When you are trail riding it doesn't matter so much, although good practice to ask for different leads, because horses do tend to prefer one to the other.
As for lead changes, well a simple change, you come down out of the canter into trot, and then ask for the canter again on the other lead.
Then there is a flying change, much more fun, asking the horse to change leads without falling back into trot...
Here is a link to a video, that may help, and a whole bunch of other suggested videos on the side.
DISCLAIMER:Had to go do stuff and did not post this, so could be I'm repeating stuff, so is so sorry.